Agassi was a much better "agressive" baseliner but a much weaker defender, i think Nole will finish slightly higher overall (slam count, MS, titles, number one's weeks...)

If Sampras was here along with Federer, Nadal, Murray and Djokovic, it would be a ridiculous tough era, nobody would win much

AO: 40/60 against the top 4, very tough ask for Pistol Pete
French: 0 chance
Wimbledon: in the current speed, it would be 50/50 against Federer, 65/35 against Murray, 70/30 against Nadal, 80/20 against Djokovic, he would win some titles there, but not 6/7 for sure
US Open: 45/55 against Federer, 55/45 against Djoker, 60/40 against Murray and Nadal, still tough.

We can compare them but it is just speculation, IMO Agassi,Sampras,Federer all of them are great shotmakers nothing like Murray,Djokovic,Nadal. Just look at Federer 31 have been in every final of all GSs at least 5 times.If that is not greatness i do not know what it is then,
I find out Murray/Djokovic and Djokovic/Nadal matches very very boring. Drama yes but tennis wise BS. Agassi and Sampras will be good in this era as well.

Sampras wouldn't win many slams. Maybe 1-2 tops and also reach another 2-3 Wimbledon finals. His style would be obsolete and no amount of "talent" can compensate for slow surfaces and fluff balls.

Agassi on the other hand, he might have had a shot at winning 4-5

I think the opposite, yes, Agassi is more suited for these slow courts but his game would be nothing special today and he would have a hard time for breaking the unbelievable defense of the top 4 two time in a row.

Sampras would still have some huge weapons, if he's on, even in these slow courts, he would be very hard to beat.